Radiation curable polymer dispersions and in particular polyurethane dispersions are known since long to provide coatings for different substrates such as wood, plastics and metal which show a good mechanical and chemical resistance as well as some flexibility.
Ethylenically unsaturated polyurethanes have been made from the end capping of an isocyanate terminated polyurethane pre-polymer with an ethylenically unsaturated, especially (meth)acrylated monomer. Ethylenically unsaturated polyurethanes can also be made from the chain extension of isocyanate terminated ethylenically unsaturated polyurethane pre-polymers with polyamines.
A drawback of known (meth)acryloyl functional dispersions is their limitation to provide good surface cure when irradiated with long wavelength (300 nm or higher) UV light sources such as UV-LED (light emitting diode) or UV-A lamps.
Current UV formulations curable with long wavelength UV light sources are sensitive to oxygen inhibition resulting in a reduced surface cure reactivity which makes them poorly suited for applications where high speed or low energy curing is required.
The phenomena of oxygen inhibition can be mediated by applying a nitrogen blanket, in situ oxygen barriers such as paraffin wax, oxygen scavengers or the use of higher intensity lamps. A drawback of these existing attempts to solve this problem is that each of these techniques offers limited protection against said oxygen inhibition and increases the coating cost. Furthermore, they are limited to industrial applications and are very difficult to adapt to field applied applications such as domestic and small to medium sized workshop applications.
We have now found dispersions which overcome these problems, at least in part.
EP 0 794 204 discloses aqueous polyurethane dispersions having latent crosslinking properties and based on a mixture of 2 different disperse phases. The dispersions used as coatings and in particular as adhesives are cured by drying. No radiation curable compositions are disclosed in said document.